refactor: clean up dead files, restructure nav, align QuickStartPage to monochrome

- Delete 6 unused files: ThemeToggle, themeStore, AppLayout-original,
  QuickStartPage-Enhanced, UpgradePrompt, SettingsPage
- Restructure Account/Settings navigation: merge Settings into Account page,
  make AccountSettingsPage the /account index, remove orphaned /account-settings route
- Remove "Settings" nav item (consolidated under Account)
- Add export preferences and team categories link to AccountSettingsPage
- Align QuickStartPage to monochrome design system: replace cyan/blue/violet
  accent colors with white opacity variants
- Replace non-functional search button with search spinner indicator

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/router.tsx

@@ -1,4 +1,4 @@
-import { createBrowserRouter, Navigate } from 'react-router-dom'
+import { createBrowserRouter } from 'react-router-dom'
 import { lazy, Suspense } from 'react'
 import { AppLayout, ProtectedRoute } from '@/components/layout'
 import { RouteError } from '@/components/common/RouteError'
@@ -16,7 +16,6 @@ const TreeNavigationPage = lazy(() => import('@/pages/TreeNavigationPage'))
 const TreeEditorPage = lazy(() => import('@/pages/TreeEditorPage'))
 const SessionHistoryPage = lazy(() => import('@/pages/SessionHistoryPage'))
 const SessionDetailPage = lazy(() => import('@/pages/SessionDetailPage'))
-const SettingsPage = lazy(() => import('@/pages/SettingsPage'))
 const AccountSettingsPage = lazy(() => import('@/pages/AccountSettingsPage'))
 // Admin pages
 const AdminLayout = lazy(() => import('@/components/admin/AdminLayout'))
@@ -117,22 +116,6 @@ export const router = createBrowserRouter([
           </Suspense>
         ),
       },
-      {
-        path: 'settings',
-        element: (
-          <Suspense fallback={<PageLoader />}>
-            <SettingsPage />
-          </Suspense>
-        ),
-      },
-      {
-        path: 'account-settings',
-        element: (
-          <Suspense fallback={<PageLoader />}>
-            <AccountSettingsPage />
-          </Suspense>
-        ),
-      },
       // Admin routes
       {
         path: 'admin',
@@ -215,21 +198,25 @@ export const router = createBrowserRouter([
         path: 'account',
         element: (
           <Suspense fallback={<PageLoader />}>
-            <ProtectedRoute requiredRole="owner">
-              <AccountLayout />
-            </ProtectedRoute>
+            <AccountLayout />
           </Suspense>
         ),
         children: [
           {
             index: true,
-            element: <Navigate to="/account/categories" replace />,
+            element: (
+              <Suspense fallback={<PageLoader />}>
+                <AccountSettingsPage />
+              </Suspense>
+            ),
           },
           {
             path: 'categories',
             element: (
               <Suspense fallback={<PageLoader />}>
-                <TeamCategoriesPage />
+                <ProtectedRoute requiredRole="owner">
+                  <TeamCategoriesPage />
+                </ProtectedRoute>
               </Suspense>
             ),
           },